1. Field of the Invention
The present invention relates to an industrial furnace, especially a single chamber vacuum furnace, for heat treating metal workpieces. The furnace has a heating chamber which is disposed in a furnace housing, and receives a charge; the heating chamber can be heated via heating elements, and is provided with at least one closable chamber opening through which there can be passed a cooling gas which, with the aid of a blower, can be circulated through a heat exchanger. To control the flow of cooling gas arriving through the cooling gas delivery pipe, a distribution means which can be moved back and forth during the cooling process is pivotably mounted in the region of the chamber opening provided for the entry of the cooling gas.
2. Description of the Prior Art
Industrial furnaces of this general type are used, in particular, in order to be able to harden pieces of high-speed steel tool and other tool steel. However, such furnaces are also suitable for other thermal treatments, such as for bright annealing. A furnace of this type comprises a double-walled steel housing having a front door which can be opened, thus permitting access to the heating chamber. The heating chamber comprises a steel casing which is lined with heat insulating material. The top and the bottom of the heating chamber are provided in a customary manner with a large opening for the passage of gas. During the heating and pretension cycles, these openings are closed-off by insulated blocking cover plates. The upper opening of the heating chamber for the passage of gas is connected directly via a pipe connection to the delivery pipe of a blower.
The gas stream which enters the heating chamber through the pipe connection is only in a position to flow over a relatively small charge. The charge cannot be enlarged because this would result in a reduction of the cooling rate. An approach is also not possible to enlarge the diameter of the pipe connection of the blower, because unless the capacity of the blower is changed, a loss in velocity of the cooling air would result. However, a high velocity of the cooling gas is necessary in order to achieve a rapid cooling of the charge. Accordingly, for example, there is only possible to carry out a hardening if the dissipation of heat is rapid enough. Thus, in order to achieve a rapid cooling of the charge, there is necessary to circulate the cooling gas which is blown into the heating chamber at a high velocity. At a given blower capacity, the velocity of the gas is a function of the diameter of the pipe connection; the pipe diameter, in turn, determines the size of the charge surface over which the cooling gas is to flow. Thus, in practice, the furnace output is inherently limited to a fixed grade of workpieces which are to be thermally treated.
Therefore, in order to increase the furnace output, and to better utilize the existing furnace capacity for subjecting a larger charge surface to a rapid cooling-off, German Offenlegungsschrift No. 28 44 8430 made a proposal to pivotably mount a flap or damper on the chamber opening provided for the entry of gas; this flap was intended to control the arriving flow of gas in the region of the open cross-section of the chamber opening. Such a flap only provided for a very imperfect circulation of the gas over the entire charge. Above all, the cooling gas does not flow uniformly over the surface of the charge, so that a nonuniform cooling-off results. This results in the danger of warping.
An object of the present invention, with an industrial furnace of the aforementioned general type, is to improve the pivotable distribution means in such a way that there is possible therewith for the cooling gas to flow uniformly over the surface of the charge.